1. Field of the Invention
This invention relates to an automatic gain control system suitably used for a radio receiver.
2. Description of the Prior Art
FM radio broadcasts are widely enjoyed for their high sound quality. In view of the multiplicity of broadcasting stations now established, however, a serious problem of interference arises in which the sound quality of the desired channel of broadcasting is adversely affected by the transmissions of other stations. Especially, intermodulation distortion, which causes uncomfortable noises such as birdy noise resulting from the beating of a second signal with the desired receiving signal, is required to be reduced to the greatest extent possible.
Intermodulation distortion is a phenomenon caused by the nonlinear characteristics of an amplifier element or a mixing element of a receiver, which elements generate a spurious frequency spectrum, by interference between two or more different electric waves, as if there exists another electric wave at that beat frequency different from the frequencies of the first-mentioned electric waves. Assume, for instance, that an amplifier element has a nonlinear transmission characteristic as shown by equation (1) below. EQU I=a.sub.0 +a.sub.1 v+a.sub.2 v.sup.2 +a.sub.3 v.sup.3 + . . . (1)
where
I: output current, and PA1 v: input voltage PA1 .omega..sub.1 : 2.pi.f.sub.1, and PA1 .omega..sub.2 : 2.pi.f.sub.2
If there are two input signals whose frequencies are f.sub.1 and f.sub.2 in the equation (1), EQU I=a.sub.0 +a.sub.1 (v.sub.1 sin .omega..sub.1 t+v.sub.2 sin .omega..sub.2 t)+a.sub.2 (v.sub.1 sin .omega..sub.1 t+v.sub.2 sin .omega..sub.2 t).sup.2 +a.sub.3 (v.sub.1 sin .omega..sub.1 t+v.sub.2 sin .omega..sub.2 t).sup.3 ( 2)
where v.sub.1 and v.sub.2 are signal voltages of f.sub.1 and f.sub.2, respectively,
The fourth term, namely, the cubic term of equation (2) above is expanded by use of a trigonometric formula. EQU 3/4a.sub.3 v.sub.1.sup.2 v sin 2.pi.(2f.sub.1 -f.sub.2)t (3) EQU 3/4a.sub.3 v.sub.1 v.sub.2.sup.2 sin 2.pi.(2f.sub.2 -f.sub.1)t .sub.1 ( 4)
Equations (3) and (4) involve spectra of (2f.sub.1 -f.sub.2) and (2f.sub.2 -f.sub.1). In this case, if f.sub.2 is considered equal to f.sub.1 +.DELTA.f, the spectra of equations (3) and (4) occur at f.sub.1 -.DELTA.f and f.sub.1 +2.DELTA.f respectively as shown by dashed lines in FIG. 1. When the receiving frequency of a receiver is tuned to f.sub.1 -.DELTA.f or f.sub.1 +2.DELTA.f, therefore, a mixture of broadcastings of f.sub.1 and f.sub.2 is undesirably received even in the absence of a genuine signal at these tuning points.
One method to solve this problem is described below with reference to FIG. 2. In a radio frequency circuit of a radio receiver, elements having little nonlinear characteristics are used for a radio frequency amplifier ccircuit 3 and a mixer circuit 6, or the selectivity of the frequency selector circuit 2 or 4 is improved, thereby reducing the level of the interference wave applied to the mixer circuit 6. These methods have a limitation in interference wave level control, however, because these methods affect the receiving characteristics for weak input called a usable sensitivity.
Another suggested method is to control the input level of the radio frequency amplifier circuit 3 and the mixer circuit 6 by an AGC circuit 9. This method, however, does not substantially prevent the above-mentioned interference. When the receiving frequency of the receiver is tuned to, for example, f.sub.1 -.DELTA.f in FIG. 1, the intermediate frequencies of the outputs of the mixer circuit 6 present the same spectrum as the spectrum of FIG. 1. That is, the spectrum of FIG. 1 may be regarded as the spectrum of the intermediate frequencies corresponding to the respective radio frequencies. Thus, the intermediate frequencies for the interference waves f.sub.1 and f.sub.1 +.DELTA.f are displaced by .DELTA.f and 2.DELTA.f, respectively from the intermediate frequency of the receiving wave of f.sub.1 and hence may be attenuated by the intermediate frequency filter 7. As a result, it is difficult to obtain a sufficient AGC voltage by the electric waves causing the interference. The band-width of the intermediate frequency filter 7 is determined so as to prevent the affect of adjacent broadcast waves, and therefore it is substantially impossible ot obtain an AGC voltage by the interference wave. Thus, this expedient is almost of no use for preventing the intermodulation.